Slab-on-Grade control joints - align with columns?

[JAE]

Typically, consistent with the rest of the universe, we use diamond shaped block-outs in interior building slabs at columns to separate the column from the slab.
The diamond shape also allows slab control joints to intersect at the diamond corners, eliminating re-entrant corners and cracking from such corners.

In a project we have going, there is a need to align slab control joints with tile flooring joints. The tile, of course, will not align with our irregular column spacing so we are thinking of possibly laying down a control joint pattern that is independent of the columns so we can align the joints.

If we do that, we now have columns (steel tubes - 6 to 10" in size) extending through holes in the slab and we are concerned that this might cause cracking off the re-entrant corners of the "little" holes that are generally square in shape.

Has this been done before? Are the column holes small enough that re-entrant corner cracking doesn't occur? Can you use circular blockouts around the columns to avoid the corners and cracking?

Looking for ideas and experience - thanks.

 

[ron9876]

JAE my guess is that it will crack at the holes. However if the architect needs to do something different you can add reinforcement around the holes to control the crack widths.

 

[ajk1]

JAE: I don't see why you could not do cicular openings at the columns (one of our typical details has this), but that won't help with the fact that the column will restrain the slab from movement once the circular boxout is filled in. In my area (Toronto) the slab does not see significant drying shrinkage shortening till after a winter with the heating system running, when the ambient air is at its lowest RH. If the columns are restraining the slab from shrotening, then I would expect some cracking. Sorry I can't be of more help.

 

[JAE]

We were thinking also of just wrapping the columns themselves with EJ material and then reinforcing around the columns to minimize crack widths.

The problem is that the column spacing is totally independent of this large-format (24" x 48" porcelain) tile.

 

[structSU10]

Is there any wrapping around the columns? I'm used to a fairly substantial architectural finish around the column, within which the tile doesn't need to be placed, or could be cut up to match cj's at the least. Perhaps you have a different case, but I would think you could put a cj at the perimeter of this and have the tile cut so it matches this profile somehow.

If that is not the case, then the extra reinforcement for crack control is the only other option I can think of at this moment.

 

[dhengr]

JAE:
Wow.... “large-format (24" x 48" porcelain) tile.” I suspect that big a brittle tile could have other cracking problems also, namely: cracking due to differential expansion or contraction btwn. the tile and the slab, either temp. or moisture induced; and if this tile isn’t perfectly set and bedded it could be subject to flexural cracking too. 2'x4' is a big bed to get perfect for each tile, and then that tile set perfectly, and then adjacent tiles to match too. I’ve seen tile over a void in the bedding punch right through under traffic. A smaller format tile is much more forgiving. And, you shouldn’t be blamed for any problems in this regard. Have you talked with the tile manuf’er. about these types of issues, for your own protection? How are you going to detail the top slab edge at a control joint, to support the tile edge? You might be better off to cast a .75 - 1" void around the columns and fill that with caulking/filler at a later date. Also, don’t let the slab bond to/around A.B’s. or nuts as this could be a crack starter. Seems to me that the Arch. is looking for trouble on this floor system, and you should put as much of that on him as is possible. How is he going to get those tiles around the columns without some unwanted joints?

 

[haynewp]

I have used circular blockouts around columns before and did not worry about cracking around the columns. The rest of the joints on that project landed wherever the contractor wanted to place them (we gave a maximum joint spacing in each direction for him to follow, no special tile considerations). But I typically show all joints for a slab and use diamond blockouts also.

 

[Ron]

JAE...I agree with the circular blockouts, making them as small as possible. Reinforce with ring rebar.

 

[hokie66]

I tend not to align joints with columns. Best to have the columns in the centre of panels so any restraint drags the edges toward the column. Blockouts cause as much or more problems with tiling as do cracks. I would provide the joints where the architect wants, and just provide straight bars across the 4 diagonals to control reentrant corner cracking.

 

[slickdeals]

On a side note, I recently visited a project which had issues related to the diamond isolation joint. The project was founded on expansive soil and there was differential movement at these joints, causing a significant sinking (about 3/4" to 1"), which took the vinyl flooring down with it as it was bridged over the joint.

Maybe it might have been better to use a "pinwheel" type of joint in this circumstance to alleviate cosmetic damage to this joint due to differential movement.

 

[ToadJones]

On some of he commercial tile jobs I've worked in my bob-the-builder days, we cut control joints right thru the tile after the tile was installed. These joints were (supposedly) lined up with the joints in the floor below and filled with flexible sanded caulk to match the grout.

 

[dik]

What is the actual treatment of the tile at the column location? are they cut to fit around the columns with a joint? just a tile mortar joint (smaller)? or what?

I would have the slab jointing pattern mimic the tile pattern as close as possible.

What soil? any differential movement? Can you use a structured slab? added cost would be small considering the floor treatment.

Dik

 

[dik]

Can you make the tile joint closest to the column to coincide with the pile cap?

Dik

 

[OHIOMatt]

This project sounds like it may be a good candidate for shrinkage compensating concrete and large joint free expanses. It is a speciallty design and install, but may work well for this project.

 

[JAE]

Thanks for the replies.

I think we will be wrapping the columns with 1/2" expn. joint material and then aligning slab joints with tile joints (i.e. the contractor will work out the alignment not us).

We also show additional ring reinforcement around the columns so if there are any projecting cracks from the small column "hole" we at least control their width a bit.

We'll see how this works.

 

[dik]

With CAD you can prepare a drawing showing the individual tiles for the contractor to use to lay out the jointing.

Dik

 

[JAE]

Dik - we aren't going to do that. Waste of time in my view since the contractor, architect, tile mfr., etc. is going to lay it all out anyway and we are simply accommodating their recommendation to align the joints. I don't want to get into laying out tile and thus adding to the perception that we are responsible for the tile cracks. Plus we aren't paid to do that anyway.

 

[haynewp]

Is the ring reinforcing usually needed? The time I used the circular blockouts we just had the circular form and no ring reinforcing around the area. I don't think I have previously noticed cracks coming from circular holes in slabs on grade when plumbing pipes are installed either.

 

[dik]

Sorry... I should have said that a CAD drawing can be prepared showing the 'exact' location of the tiles and joints... This can be used to locate the sawcut locations... and can be done by the Contractor and/or Architect... If I were the Contractor, I'd be requesting a location of the sawcut joints... just to get me off the hook, in particular, if it is critical.

Dik